NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-18-2355-2018Growth of a sinkhole in a seismic zone of the northern Apennines (Italy)Growth of a sinkhole in a seismic zone of the northern Apennines (Italy)La RosaAlessandroalessandro.larosa@unifi.ithttps://orcid.org/0000-0003-1858-1109PagliCarolinahttps://orcid.org/0000-0002-9072-3004MolliGiancarloCasuFrancescohttps://orcid.org/0000-0001-8555-6494De LucaClaudioPieroniAmerinoD'Amato AvanziGiacomoDipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira, 4, 50121 Florence, ItalyDipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 53, 56126 Pisa, ItalyCNR, Consiglio Nazionale delle Ricerche, Istituto per il Rilevamento Elettromagnetico dell'Ambiente (IREA-CNR), Via Diocleziano, 328, 80124 Naples, ItalyPro.Geo. s.r.l. Via Valmaira, 14, 55032, Castelnuovo di Garfagnana, ItalyAlessandro La Rosa (alessandro.larosa@unifi.it)12September20181892355236619March201826March201831July201817August2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://nhess.copernicus.org/articles/18/2355/2018/nhess-18-2355-2018.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/18/2355/2018/nhess-18-2355-2018.pdf
Sinkhole collapse is a major hazard causing substantial social and
economic losses. However, the surface deformations and sinkhole evolution are
rarely recorded, as these sites are known mainly after a collapse, making the
assessment of sinkhole-related hazard challenging. Furthermore, more than
40 % of the sinkholes of Italy are in seismically hazardous zones; it
remains unclear whether seismicity may trigger sinkhole collapse. Here we use
a multidisciplinary data set of InSAR, surface mapping and historical records
of sinkhole activity to show that the Prà di Lama lake is a long-lived
sinkhole that was formed in an active fault zone and grew
through several events of unrest characterized by episodic subsidence and
lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence
at rates of up to 7.1 mm yr-1 occurred during 2003–2008, between events
of unrest. Earthquakes on the major faults near the sinkhole do not
trigger sinkhole activity but low-magnitude earthquakes at 4–12 km depth
occurred during sinkhole unrest in 1996 and 2016. We interpret our
observations as evidence of seismic creep at depth causing fracturing and
ultimately leading to the formation and growth of the Prà di Lama
sinkhole.
Introduction
Sinkholes are closed depressions with internal drainage typically associated
with karst environments, where the exposed soluble rocks are dissolved by
circulating groundwater (dissolution sinkholes) but other types of sinkholes
also exist. Subsidence sinkholes, for example, can form for both internal
erosion and dissolution of covered layers leading to downward gravitational
deformations such as collapse, sagging or suffosion (Ford and Williams, 2007;
Gutiérrez et al., 2008). Deep sinkholes have been often observed along
seismically active faults indicating a causal link between sinkhole formation
and active tectonics (Faccenna et al., 1993; Harrison et al., 2002; Closson
et al., 2005; Florea, 2005; Del Prete et al., 2010; Parise et al., 2010;
Wadas et al., 2017). In some cases, the processes responsible for their
formation have been attributed to fracturing and increased permeability in
the fault damage zone, promoting fluid circulation and weathering of soluble
rocks at depth. Additionally, when carbonate bedrocks lie below thick
non-carbonate formations, stress changes caused by faulting may cause
decompression of confined aquifers favouring upward migration of deep fluids,
hence promoting erosion and collapses (e.g. Harrison et al., 2002; Wadas et
al., 2017). Seismically induced stress changes could also trigger the collapse of
unstable cavities as in the case of the two sinkholes that formed near Ein Gedi (Dead Sea) following the Mw 5.2 earthquake on the Dead Sea
Transform fault in 2004 (Salamon, 2004). Sinkhole subsidence and collapses
are a major hazard and cause substantial economic and human losses globally
(Frumkin and Raz, 2001; Closson, 2005; Wadas et al., 2017).
In Italy, a total of 750 sinkholes have been identified and the 40 % of
them are along active faults (Caramanna et al., 2008) but this number could be
underestimated due to the high frequency of sinkholes both related to karst
and anthropogenic origin (Parise and Vennari, 2013). Seismicity-induced sinkhole deformation
has been often observed in Italy (e.g. Santo et al., 2007; Parise et al., 2010; Kawashima et al., 2010).
The sinkhole of Prà di Lama, near the village of Pieve Fosciana (Lucca
province, Italy), is a quasi-circular depression filled by a lake. Prà di
Lama is located in the seismically active Apennines range of northern
Tuscany, at the intersection of two active faults (Fig. 1). Hot springs are
also present at Pieve Fosciana suggesting that fluid migration occurs along
the faults planes. Sudden lake-level changes of up to several metres, ground
subsidence, surface fracturing and seismicity have occurred repeatedly since
at least 991 AD (Nisio, 2008). The most recent deformation events occurred
in March 1996 and between May 2016 and October 2017. However, the processes
that control the growth of the Prà di Lama sinkhole remain unclear.
Furthermore, whether seismicity along the active faults around Prà di
Lama may trigger sinkhole subsidence or collapse is debated.
In this paper we combine recent InSAR observations, seismicity and surface
mapping, as well as historical records of lake-level changes and ground
subsidence at the Prà di Lama from 1828 to understand the mechanisms of
sinkhole growth in an active fault system.
Geological setting
The area of the Prà di Lama sinkhole is located within the Garfagnana
basin (Fig. 1), an extensional graben in the north-western Apennines, a
NW–SE-trending fold-and-thrust belt formed by the stack of different tectonic
units caused by the convergence of the Corsica–European and Adriatic plates. The
current tectonic regime of the Apennines is characterized by shortening in
the eastern sector of the Apennines range and extension in the westernmost
side of the range (Elter et al., 1975; Patacca and Scandone, 1989; Bennett et
al., 2012). The contemporaneous eastward migration of shortening and upper
plate extension are believed to be caused by the roll-back subduction during
the anticlockwise rotation of the Adriatic plate (Doglioni, 1991; Meletti et
al., 2000; Serpelloni et al., 2005; Faccenna et al., 2014; Le Breton et al.,
2017). Extension started 4–5 Ma, leading
to the formation of several NW–SE-oriented grabens, bounded by NE-dipping and
SW-dipping normal faults that are dissected by several NE-trending,
right-lateral strike-slip faults (Fig. 1). The inner northern Apennines are a
seismically active area, where several earthquakes with Mw>5
occurred, including the largest instrumentally recorded earthquake,
Mw 6.5, in 1920 (Tertulliani and Maramai, 1998; Rovida et al.,
2016; Bonini et al., 2016) and the most recent Mw 5.1 earthquake
in 2013 (Pezzo et al., 2014; Stramondo et al., 2014; Molli et al., 2016).
Study area: the Prà di Lama sinkhole is marked by the yellow
star. Black tick lines are faults. Blue dots are the earthquakes between 1986
and 2017. Focal mechanisms are from the Regional Centroid Moment Tensor
(RCMT) catalogue. The yellow circles represent the areas with radii of 3 and
10 km used for the seismicity analysis. The red dot is the
Camaiore sinkhole (Buchignani et al., 2008; Caramanna et al., 2008). The red box in
the inset marks the location of the area shown in the main figure.
Geological setting of the study area. (a) Geological,
structural and geomorphological map of the area nearby Prà di Lama
showing the main tectonic and lithostratigraphic units.
(b) Schematic sedimentary sequence of the Villafranchian deposits
obtained from the well drilled at Prà di Lama (modified from Chetoni,
unpublished data). (c) Stratigraphic cross section across the
Garfagnana graben.
The uppermost stratigraphy at Prà di Lama consists of 8 m thick layer of
alluvial and palustrine gravels and sandy deposits containing peaty levels,
covering an ∼85 m thick sandy-to-silty fluviolacustrine deposits with
low permeability (from Villafranchian to present age) (Chetoni, unpublished
data) (Fig. 2a and b). These deposits cover a ∼1000 m thick turbiditic
sequence (Macigno Formation). Below it, a sequence of carbonate rocks
pertaining to the Tuscan Nappe unit is present reaching down to a depth of
∼2000 m, where anhydrites (Burano Formation) and calcareous–dolomitic
breccias (Calcare Cavernoso Formation) overlie the Tuscan Metamorphic units
(Fig. 2c).
The Prà di Lama lake lies at the centre of a depression (Figs. 2 and 5).
The low slopes characterizing the topography of the area result in the
absence of active gravitational ground motions (Fig. 2). Furthermore, the
Prà di Lama sinkhole is an isolated feature in the region, being the only
mapped sinkhole in the entire Garfagnana graben (Caramanna et al., 2008); the
closest sinkhole is in Camaiore (Buchignani et al., 2008) near the Tuscany
coast (Fig. 1).
The Prà di Lama sinkhole is located at the intersection between two
seismically active faults: the Corfino normal fault (Di Naccio et al., 2013;
ISIDe working group, 2016) and the right-lateral strike-slip
M. Perpoli–T. Scoltenna fault that recently generated the Mw 4.8
earthquake in January 2013 (Fig. 1) (Pinelli, 2013; Molli et al., 2017). Hot
water springs are also present at Prà di Lama (Bencini et al., 1977;
Gherardi and Pierotti, 2018). Geochemical analyses of the Prà di Lama
spring waters by Gherardi and Pierotti (2018), expanding on previous research
(Baldacci et al., 2007), suggest that both shallow and deep aquifers are
present below Prà di Lama (Fig. 2b). Shallow aquifers have low salinity
and low temperature, while waters feeding the thermal springs have high
temperature (∼57∘C) and high salinity (5.9 g kgw-1),
suggesting the presence of a deep aquifer at ∼2000 m into the
anhydrite and the calcareous–dolomitic breccia. The high salinity of the
deep groundwaters is associated with dissolution of the deep evaporitic
formations. Furthermore, unmixing of deep and shallow waters is interpreted
by Gherardi and Pierotti (2018) as evidence of their rapid upwelling, likely
occurring along the existing faults.
Data
Century-scale historical records of sinkhole activity are available at
Prà di Lama and allow us to determine the timescale of sinkhole
evolution as well as to characterize the different events of unrest, in
particular the two most recent events in 1996 and 2016. InSAR time series
analysis is also carried out to measure ground deformations in the Prà
di Lama sinkhole in the time period between events of unrest. Finally, the
local catalogue of seismicity (ISIDE catalogue, INGV) is used to inform us
on the timing and types of brittle failures in the area of the sinkhole.
Evolution of the Prà di Lama lake between 1994 and 2017. Lake
shore variation has been retrieved from the analysis of the Landsat image.
Historical record
The first historical record of the Prà di Lama sinkhole dates back to
the 991 AD, when the area was described as a seasonal shallow pool fed by
springs. Since then, the depression grew and several events of unrest
consisting of fracturing and fluctuations of the lake level were reported
(Raffaelli, 1869; De Stefani, 1879, Giovannetti, 1975) (Table 1). In particular, eight events of unrest were reported, giving an
average of 1 event of unrest every 26 years. We conducted direct observation
of surface deformation around the lake for the two most recent events in
1996 and 2016.
In 1996, the lake level experienced a fall of up to 4 m (Figs. 3 and S1 in the Supplement)
and at the same time the springs outside the lake suddenly increased the
water outflow. Clay and mud were also ejected by the springs outside the
lake, while fractures and slumps occurred within the lake due to the water
drop (Figs. 3 and S1). The unrest lasted approximately 2 months, from
March to April 1996. During the final stages, the water level in the lake
rose rapidly, recovering its initial level, and contemporaneously the
spring water flow reduced.
In June 2016, an event of unrest consisting of ground subsidence on the
western and southern sides of the Prà di Lama lake started and lasted
approximately 9 months, until February 2017. During this period fractures
formed and progressively grew, increasing their throw to up to 70 cm and
affecting a large area on the western side of the lake (Figs. 3 and S2).
Subsidence around the lake resulted in an increase in the lake surface, in
particular on the western side and in the formation of tensile fractures
(Figs. 3 and S2). Unlike the 1996 events of unrest, no lake-level
changes or increase in water flow were
observed from the springs around the lake.
Description of the activity at Prà di Lama lake.
YearBrief description of the event991Seasonal pool fed by springs.1828Bursts of the spring water flow. Uprising of muddy waters and clays (Raffaelli, 1869; De Stefani, 1879).1843Bursts of the spring water flow. Uprising of muddy waters and clays (Raffaelli, 1869; De Stefani, 1879).1876Subsidence and fracturing (De Stefani, 1879).1877Subsidence and fracturing (De Stefani, 1879).1962Bursts of the spring water flow. Uprising of muddy waters and clays (Giovannetti, 1975).1969Abrupt falling of the water level and fracturing along the shores. The lake almost disappeared (Giovannetti, 1975).1985Arising of muddy waters in a well.1996Abrupt fall of the water level and fracturing along the shores.2016–2017Subsidence and fracturing.InSAR
InSAR is ideally suited to monitor localized ground deformation such as that
caused by sinkholes, as it can observe rapidly evolving deformation of the
ground at high spatial resolution (Baer et al., 2002; Castañeda et al., 2009; Atzori et al., 2015; Abelson et al., 2017). Furthermore, the availability of
relatively long data sets of SAR images in the Apennines allows us to study
the behaviour of the Prà di Lama sinkhole using multi-temporal
techniques. We processed a total of 200 interferograms using SAR images
acquired by the ENVISAT satellite between 2003 to 2010 from two distinct
tracks in ascending or descending viewing geometry (tracks 215 and 437). We
used the Small BAseline Subset (SBAS) multi-interferogram method originally
developed by Berardino et al. (2002) and recently implemented for parallel computing processing
(P-SBAS) by Casu et al. (2014) to obtain incremental and cumulative time series of InSAR
line-of-sight (LOS) displacements as well as maps of average LOS velocity.
In particular, the InSAR processing has been carried out via the ESA
platform P-SBAS open-access online tool named G-POD (Grid Processing On
Demand) that allows ground displacement time series to be generated from a set of
SAR data (De Luca et al., 2015).
The P-SBAS G-POD tool allows the user to set some key parameters to tune the
InSAR processing. In this work, we set a maximum perpendicular baseline
(spatial baseline) of 400 m and maximum temporal baseline of 1500 days. The
geocoded pixel dimension was set to ∼80 m by 80 m
(corresponding to averaging together 20 pixels in range and 4 pixels in
azimuth).
We initially set a coherence threshold to 0.8 (0 to 1 for low to high
coherence) in order to select only highly coherent pixels in our
interferograms. The 0.8 coherence threshold is used to select the pixels for
the phase unwrapping step that is carried out by the extended minimum cost
flow (EMCF) algorithm (Pepe and Lanari, 2006). By setting high values of this
parameter the pixels in input to the EMCF algorithm are affected by less
noise compared to selecting low values, thus increasing the quality of the
phase unwrapping step itself and
reducing the noise in our final velocity maps and time series (De Luca et
al., 2015; Cignetti et al., 2016).
We also inspected the series of interferograms and excluded individual
interferograms with low coherence. We identified and discarded 29 noisy
interferograms in track 215A and other 11 interferograms in track 437D.
Finally, we applied an atmospheric phase screen (APS) filtering to mitigate
further atmospheric disturbances (Hanssen, 2001). Accordingly, we used a
triangular temporal filter with a width of 400 days to minimize temporal
variations shorter than about a year as we focus on steady deformations
rather than seasonal changes. A shorter time interval of 300 days was also
tested but provided more noisy time series.
The average velocity map and the incremental time series of deformation
obtained with the P-SBAS method have to be referred to a stable reference
point. For our analysis, the reference point was initially set in the city
of Massa because GPS measurements from Bennett et al. (2012) show that the surface velocities
there are <1 mm yr-1; therefore, Massa can be considered
stable. Assuming Massa as a reference point, the average velocity map revealed
the deformation pattern around the Prà di Lama lake. We then moved the
reference point outside the sinkhole deformation pattern but close to the
village of Pieve Fosciana (Fig. S3a). Selecting a reference point close to
our study area rather than in Massa allowed us to better minimize the
spatially correlated atmospheric artefacts.
(a, b) Maps of average surface velocity and its
vertical (c) and east–west (d) components obtained from
ENVISAT SAR images acquired between 2003 and 2010. Negative values indicate
a range increase. The white line in panel (a) marks the cross section
shown in Fig. 4. The black star is the point used as a reference for the
InSAR-SBAS processing. (e–h) Time series of incremental deformation
extracted from the pixel bounded with the white rectangle.
As a final post-processing step we also calculated the vertical and
east–west components of the velocity field in the area covered by both the
ascending and descending tracks and assuming no north–south displacement.
Given that the study area is imaged by the ENVISAT satellite from two
symmetrical geometries with similar incidence angles (few degrees of
difference), the vertical and east–west components of the velocity field can
simply be obtained solving the following system of equations (Manzo et al., 2006):
vH=cosϑsin(2ϑ)vDESC-vASC=vDESC-vASC2sinϑvV=sinϑsin2ϑvDESC+vASC=vDESC+vASC2cosϑ,
where vH and vV are the horizontal and vertical component of the
velocity field, vDESC and vASC are the average LOS velocities in
the descending and ascending tracks, respectively; ϑ is the
incidence angle.
Cross section of topography and InSAR velocities along the A-A′
profile as shown in Fig. 3a.
The InSAR P-SBAS analysis shows that significant surface deformation occurs
at Pieve Fosciana between 2003 and 2010. The observed deformation pattern
consists of a range increase mainly on the western flank of the Prà di
Lama lake. The range increase is observed in both ascending and descending
velocity maps (Fig. 4a, b), with average LOS velocities of up to -7.1 mm yr-1 decaying to -1 mm yr-1 over a distance of 400 m away from
the lake. Elsewhere around the lake coherence is not kept due to the
presence of both cropland and woodland cover, leading to decorrelation.
However, few coherent pixels are identified on the eastern flank of the
lake, in areas with buildings and sparse vegetation cover, suggesting that
the deformation pattern may be circular, with a radius of ∼600 m (Figs. 4 and 5). In order to increase the number of analysed pixels we
tested decreasing our coherence threshold from 0.8 to 0.4. The results are
displayed in Fig. S3b and show that only a few more pixels are gained north
of the sinkhole compared to choosing a threshold of 0.8 (Fig. 4). We
conclude that decreasing the coherence threshold does not allow
the entire deformation pattern to be retrieved, likely due to the fact the area is
vegetated.
The maps of vertical and east–west velocities show vertical rates of -4.6 mm yr-1 and horizontal eastward velocities of 5.4 mm yr-1 (Fig. 4c,
d) consistent with subsidence and contraction centred at the lake.
Furthermore, Fig. 5 shows that the current deformation pattern follows the
topography, suggesting that subsidence at Prà di Lama is a long-term
feature. The time series of cumulative LOS displacements show that
subsidence occurred at an approximately constant rate between the 2003 and
the 2008 but it slowed down in 2008 (Fig. 4e, f), indicating that subsidence
at Prà di Lama occurs also between events of unrest. Furthermore, our
time series of vertical and east–west cumulative displacements also confirm
that the fastest subsidence and contemporaneous eastward motion occurred
until 2008 (Fig. 4g, h). In order to better understand the mechanisms
responsible for the sinkhole growth and the different types of episodic
unrest we also analysed the seismicity.
Seismicity
Seismicity at the Prà di Lama lake was analysed using the catalogue
ISIDe (Italian Seismological Instrumental and Parametric Data-Base) spanning
the time period from 1986 to 2016. We calculated the cumulative seismic
moment release using the relation between seismic moment and magnitudes
given by Kanamori (1977). First, we analysed the seismic moment release and the
magnitude content of the earthquakes in the area encompassing the sinkhole
and the fault intersection (10 km radius, Fig. 1) to understand whether
unrest at Prà di Lama is triggered by earthquakes along the active
faults (Fig. 6). Figure 6a shows that, although several seismic swarms
occurred in the area, no clear temporal correlation between the swarms and
the events of unrest at Prà di Lama is observed, suggesting that the
majority of seismic strain released on faults around the Prà di Lama
lake does not affect the activity of the sinkhole. We removed from the plot
in Fig. 6a the high-magnitude earthquake, Mw 4.8, on the 25 January 2013 in order to better visualize the pattern of seismic moment
release in time. In any case, no activity at Prà di Lama was reported in
January 2013.
Seismicity of an
area 10 km in radius around the Prà di Lama lake. Cumulative seismic
moment released in the area (a) and histograms of the number of
earthquakes per month. Three different classes of magnitude have been
created: Ml<2.0(b), 2.0<Ml<3.0(c) and Ml>3.0(d). The data set covers the period between 1986 and 2017. The
red transparent bars indicate the two events of unrest of 1996 and 2016. The
orange solid line in panel (a) marks the Mw 4.8
earthquake, on 25 January 2013.
Seismicity of an area 3 km in radius around the Prà di Lama
lake. Plot of the cumulative seismic moment released in the area (a)
and histograms showing the number of earthquakes occurred each month. Two
different classes of magnitude have been created: Ml<2.0(b), 2.0<Ml<3.0(c). No events of
Ml>3.0 occurred in the area between 1986 and 2017. The red
transparent bars indicate the two events of unrest of 1996 and 2016.
Comparison between the earthquakes (blue lines) in the Garfagnana
area (INGV Catalogo Paramentrico dei Terremoti Italiani CPTI15, Rovida et
al., 2016) and events of unrest at the Prà di Lama sinkhole (red
lines). Light grey shading marks the time period covered by CPT15, while dark grey shading is the period covered by ISIDe.
We also analysed the local seismicity around the Prà di Lama lake, within
a circular area of 3 km radius around the lake (Fig. 1) to better understand
the deformation processes occurring at the sinkhole and we found that swarms
of low-magnitude earthquakes (Ml≤2) occurred during both
events of unrest at Prà di Lama in 1996 and 2016 (Fig. 7a, b, c), while a
few earthquakes with magnitudes >2 occurred irrespective of the events of
unrest. This indicates that seismicity during sinkhole activity is
characterized by seismic energy released preferentially towards the lower end
of the magnitude spectrum. This pattern is specific to the sinkhole area,
because in the broader region (Fig. 6b, c) the majority of earthquake
magnitudes are in the range Ml>2 to Ml<3 and few
with Ml>3 also occurred. We also analysed the hypocentres of
the earthquakes around the Prà di Lama lake (3 km radius) and find that
these range between 4.5 and 11.5 km depth, indicating that deformation
processes in the fault zone control the sinkhole activity. On the other hand,
no earthquakes were recorded at Prà di Lama during the period of
subsidence identified by InSAR between 2003 and 2010, indicating that
subsidence between events of unrest continues largely aseismically.
To strengthen our seismicity analysis and clarify whether a connection
between major tectonic earthquakes and sinkhole unrest exists, we also
analysed the historical parametric seismic catalogues (Rovida et al., 2016;
INGV Catalogo Parametrico dei Terremoti Italiani, CPTI15). Figure 8 shows the
occurrence of major earthquakes, with magnitude >4.0 up to 20 km away
from Pieve Fosciana and the events of sinkhole unrest at Prà di Lama. No
clear connection between occurrence of large distant earthquakes and events
of sinkhole unrest is observed, suggesting that the mechanisms responsible
for the activation of the Prà di Lama sinkhole should be attributed to local
processes.
Discussion and conclusions
A multi-disciplinary data set of InSAR measurements, field observations and
seismicity reveal that diverse deformation events occur at the Prà di
Lama sinkhole. Two main events of sinkhole unrest occurred at Prà di
Lama in 1996 and 2016 but the processes had different features. In 1996 the
lake level dropped together with increased water outflow from the springs,
while in 2016 ground subsidence led to the expansion of the lake surface and
fracturing. In 2016, fractures formed on the south-western shore of the
lake. The main active strike-slip fault is also oriented SW, suggesting a
possible tectonic control on the deformation.
We considered processes not related to the sinkhole activity that could
explain the observed deformation at Prà di Lama. Active landslides can
cause both vertical and horizontal surface motions (e.g. Nishiguchi et al.,
2017). However, no landslides are identified in the deforming area around the
sinkhole (Fig. 3). Furthermore, the low topographic slopes rule out the
presence of active landslides in the area. Concentric deformation patterns
are observed above shallow aquifers (e.g. Amelung et al., 1999). However,
deformation caused by aquifers have a seasonal pattern rather than continuous
subsidence over the timespan of several years, as in Prà di Lama. A
long-term subsidence could only be caused by over-exploitation of an aquifer
but no water is pumped from the aquifers in the deforming area around Prà
di Lama. We conclude that the observed InSAR deformation is caused by the
sinkhole.
InSAR analysis shows that continuous but aseismic subsidence of the sinkhole
occurred between the two events of unrest, during the period 2003–2010.
Instead swarms of low-magnitude earthquakes coeval to the unrest events of
1996 and 2016 were recorded at depth between 4.5 and 11.5 km, indicating
that a link between low-magnitude seismicity and sinkhole activity exists.
We suggest that seismic creep in the fault zone underneath Prà di Lama
occurs, causing the diverse deformation events.
Seismic creep at depth could have induced pressure changes in the aquifer
above the fault zone (1996 events) as well as causing subsidence by
increased fracturing (2016 events). The seismicity pattern revealed by our
analysis suggests that the Mt Perpoli–T. Scoltenna strike-slip fault system
underneath Prà di Lama is locally creeping, producing seismic sequences
of low-magnitude earthquakes. Similar seismicity patterns were observed
along different active faults (i.e. Nadeau et al., 1995; Linde et al., 1996; Rau et al., 2007; Chen et al., 2008; Harris, 2017). In 2006, along the Superstition Hills
fault (San Andreas fault system, California) seismic creep has been favoured
by high water pressure (Scholz, 1998; Wei et al., 2009; Harris, 2017). We suggest that along the fault zone below
Prà di Lama an increase in pressure in the aquifer in 1996 caused
fracturing at the bottom of the lake and upward migration of fluids rich in
clays, in agreement with the observations of lake-level drop and mud-rich
water ejected by the springs in 1996. Our interpretation is also in
agreement with geochemical data indicating that the high salinity of thermal
waters at Prà di Lama have a deep origin, ∼2000 m, where
fluid circulation dissolves evaporites and carbonates, creating cavities and
then reaching the surface by rapid upwelling along the faults system (Gherardi and Pierotti, 2018). The
presence of deep cavities and a thick non-carbonate sequence suggests that
the Prà di Lama sinkhole is a deep-sited caprock collapse sinkhole
according to the sinkhole classification of Gutiérrez et al. (2008, 2014). Sudden fracturing and periods
of compaction of cavities created by enhanced rock dissolution and upward
erosion in the fluid circulation zone could explain both sudden subsidence
and fracturing, as in 2016, and periods of continuous but aseismic
subsidence as in 2003–2010. Similar processes have been envisaged for the
formation of a sinkhole at the Napoleonville Salt Dome, where a seismicity
study suggests that fracturing enhanced the rock permeability, promoting the
rising of fluids and, as a consequence, erosion and creation of deep
cavities prone to collapse (Sibson, 1996; Micklethwaite et al., 2010; Nayak and Dreger, 2014; Yarushina et al., 2017). Recently, a sequence of seismic events was
identified at Mineral Beach (Dead Sea fault zone) and was interpreted as the
result of crack formations and faulting above subsurface cavities (Abelson et al., 2017).
Precursory subsidence of years to a few months has been observed to precede
sinkhole collapse in carbonate or evaporitic bedrocks (e.g. Baer et al.,
2002; Nof et al., 2013; Cathleen and Bloom, 2014; Atzori et al., 2015;
Abelson et al., 2017). However, the timing of these processes strongly
depends on the rheological properties of the rocks (Shalev and Lyakhovsky,
2012). Furthermore, the presence of a thick lithoid sequence in Prà di
Lama may delay sinkhole collapse, also in agreement with the exceptionally
long timescale (since AD 991) of growth of the Prà di Lama sinkhole
(Shalev and Lykovsky, 2012; Abelson et al., 2017). However, at present we are
not able to establish if and when a major collapse will occur in Prà di
Lama.
We identified a wide range of surface deformation patterns associated with
the Prà di Lama sinkhole and we suggest that a source mechanism for the
sinkhole formation and growth is seismic creep in the active fault zone
underneath the sinkhole. This mechanism could control the evolution of other
active sinkholes in Italy as well as in other areas worldwide where sinkhole
form in active fault systems (e.g. Dead Sea area). InSAR monitoring has
already been shown to be a valid method to detect precursory subsidence occurring
before a sinkhole collapse and the recent SAR missions, such as the European
Sentinel-1, will very likely provide a powerful tool to identify such
deformations.
The DEM data used in this study are from the SRTM (Shuttle
Radar Topography Mission) by JPL (NASA) and accessible at
https://earthexplorer.usgs.gov (USGS, 2018). The lidar DEM data were
taken from Regione Toscana through the GEOscopio webgis portal
(http://www502.regione.toscana.it/geoscopio/cartoteca.html; Regione
Toscana, 2018). The seismicity data are provided by the Istituto Nazionale di
Geofisica e Vulcanologia (INGV) through the Italian Seismological
Instrumental and Parametric Data-Base (ISIDe; INGV, 2016) and the Catalogo
Parametrico dei Terremoti Italiani 2015 (CPTI15) (Rovida et al., 2016).
The supplement related to this article is available online at: https://doi.org/10.5194/nhess-18-2355-2018-supplement.
ALR, CP and GM planned the work. ALR drafted the
manuscript and prepared the figures with input from all other authors.
Previously unpublished historical data from Prà di Lama were collected by
AP. ALR processed the InSAR data under the guidance of FC and CDL. ALR and CP
analysed the seismic data. ALR, GM and AP collected structural data around
the lake from the 2016 event. GD'AA and GM provided the geological,
geomorphological and structural data of the Garfagnana area.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank the two anonymous reviewers for their constructive and useful
comments. We thank the European Space Agency (ESA) for providing the ENVISAT
SAR data used in this study through the VA4. This work was partially supported by the
ESA G-POD and GEP projects. Part of the post-processing was carried out at IREA-CNR through the Infrastructure of High Technology for
Environmental and Climate Monitoring project for Structural improvement
(I-AMICA-PONa3_00363) financed under the National Operational
Programme (NOP) for “Research and Competitiveness 2007–2013” and co-funded
by European Regional Development Fund (ERDF) and national resources. All
processed interferograms are archived at IREA-CNR, Naples. Alessandro La Rosa thanks
IREA-CNR, Naples for his InSAR training internship. Carolina Pagli gratefully
acknowledges the support she received through her Rita Levi Montalcini
fellowship (MIUR Montalcini 26259_21/12/2013). This work was also financially supported by
Università di Pisa.
Edited by: Mario Parise
Reviewed by: two anonymous referees
ReferencesAbelson, M., Aksinenko, T., Kurzon, I., Pinsky, V., Baer, G., Nof, R., and
Yechieli, Y.: Nanoseismicity forecast sinkhole collapse in the Dead Sea coast
years in advance, Geology, 46, 83–86, 10.1130/G39579.1, 2017.Amelung, F., Galloway, D. L., Bell, J. W., Zebker, H. A., and Laczniak, R.
J.: Sensing the ups and downs of Las Vegas: InSAR reveals structural control
of land subsidence and aquifer-system deformation, Geology, 27, 483–486,
10.1130/0091-7613(1999)027<0483:STUADO>2.3.CO;2, 1999.Atzori, S., Baer, G., Antonioli, A., and Salvi, S.: InSAR-based modelling and
analysis of sinkholes along the Dead Sea coastline, Geophys. Res. Lett., 42,
8383–8390. 10.1002/2015GL066053, 2015.Baer, G., Schattner, U., Wachs D., Sandwell, D., Wdowinski, S., and Frydman,
S.: The lowest place on Earth is subsiding – An InSAR (Interferomeric
Synthetic Aperture Radar) Perspective, Geol. Soc. Am. Bull., 114, 12–23,
10.1130/0016-7606(2002)114<0012:TLPOEI>2.0.CO;2, 2002.
Baldacci, F., Botti, F., Cioni, R., Molli, G., Pierotti, L., Scozzari, A.,
and Vaselli, L.: Geological-structural and hydrogeochemical studies to
identify sismically active structures: case history from the Equi
Terme-Monzone hydrothermal system (Northern Apennine – Italy), Geoitalia,
6th Italian Forum of Earth Sciences, 12–14 September 2007, Rimini, Italy,
2007.
Bencini, A., Duchi, V., and Martini, M.: Geochemistry of thermal springs of
Tuscany (Italy), Chem, Geol,, 19, 229–252, 1977.Bennet, R. A., Serpelloni, E., Hreinsdottir, S., Brandon, M. T., Buble, G.,
Basic T., Casale, G., Cavaliere, A., Anzidei, M., Marjonovic, M., Minelli,
G., Molli, G., and Montanari, A.: Syn-convergent extension observed using the
RETREAT GPS network, northern Apennines, Italy, J. Geophys. Res., 117,
B04408, 10.1029/2011JB008744, 2012.Berardino, P., Fornaro, G., Lanari, R., and Sansosti, E.: A new algorithm for
surface deformation monitoring based on Small Baseline Differential SAR
interferograms, Int. Geosci. Remote Se., 40, 2375–2383,
10.1109/TGRS.2002.803792, 2002.Bonini, M., Corti, G., Donne, D. D., Sani, F., Piccardi, L., Vannucci, G.,
Genco, R., Martelli, L., and Ripepe, M.: Seismic sources and stress transfer
interaction among axial normal faults and external thrust fronts in the
northern Apennines (Italy): a working hypothesis based on the 1916–1920
time-space cluster of earthquakes, Tectonophysics, 680, 67–89,
10.1016/j.tecto.2016.04.045, 2016.Buchignani, V., D'Amato Avanzi, G., Giannecchini, R., and Puccinelli, A.:
Evaporite karst and sinkholes: a synthesis on the case of Camaiore (Italy),
Environ. Geol., 53, 1037-1044, 10.1007/s00254-007-0730-x, 2008.Caramanna, G., Ciotoli, G., and Nisio, S.: A review of natural sinkhole
phenomena in Italian plain areas, Nat. Hazards, 45, 145–172,
10.1007/s11069-007-9165-7, 2008.Castañeda, C., Gutiérrez, F., Manunta, M., and Galve, J. P.: DInSAR
measurements of ground deformation by sinkholes, mining subsidence, and
landslides, Ebro River, Spain, Earth Surf. Proc. Land., 34, 1562–1574,
10.1002/esp.1848, 2009.Casu, F., Elefante, S., Imperatore, P., Zinno, I., Manunta, M., De Luca, C.,
and Lanari, R: SBAS-DInSAR parallel processing for deformation time-series
computation, IEEE J. Sel. Top. Appl., 7, 3285–3296,
10.1109/JSTARS.2014.2322671, 2014.Cathleen, J. and Blom, R.: Bayou Corne, Louisiana, sinkhole: Precursory
deformation measured by radar interferometry, Geology, 42, 111–114,
10.1130/G34972.1, 2014.Chen, K. H., Nadeau, R. M., and Rau, R.: Characteristic repeating earthquakes
in an arc-continent collision boundary zone: The Chihshang fault of eastern
Taiwan, Earth Planet. Sc. Lett., 276, 262–272,
10.1016/j.epsl.2008.09.021, 2008.Cignetti, M., Manconi, A., Manunta, M., Giordan, D., De Luca, C., Allasia,
P., and Ardizzone, F.: Taking Advantage of the ESA G-POD Service to Study
Ground Deformation Processes in High Mountain Areas: A Valle d'Aosta Case
Study, Northern Italy, Remote Sensing, 8, 852, 10.3390/rs8100852,
2016.Closson, D.: Structural control of sinkholes and subsidence hazards along the
Jordanian Dead Sea coast, Environ. Geol., 47, 290–301,
10.1007/s00254-004-1155-4, 2005.Closson, D., Karaki, N. A., Klinger, Y., and Hussein, M. J.: Subsidence and
Sinkhole Hazard Assessment in the Southern Dead Sea Area, Jordan, Pure Appl.
Geophys., 162, 221–248, 10.1007/s00024-004-2598-y, 2005.Del Prete, S., Iovine, G., Parise, M., and Santo, A.: Origin and distribution
of different types of sinkholes in the plain areas of Southern Italy, Geodin.
Acta, 23, 113–127, 10.3166/ga.23.113-127, 2010.De Luca, C., Cuccu, R., Elefante, S., Zinno, I., Manunta, M., Casola, V.,
Rivolta, G., Lanari, R., and Casu, F.: An On-Demand Web Tool for the
Unsupervised Retrieval of Earth's Surface Deformation from SAR Data: The
P-SBAS Service within the ESA G-POD Environment, Remote Sensing, 7,
15630–15650, 10.3390/rs71115630, 2015.
De Stefani, C.: Le Acque Termali di Pieve Fosciana, Memorie della Società
Toscana di Scienze Naturali, 4, 72–97, 1879.Di Naccio, D., Boncio, P., Brozzetti, F., Pazzaglia, F. J., and Lavecchia,
G.: Morphotectonic analysis of the Lunigiana and Garfagnana grabens (northern
Apennines, Italy): Implications for active normal faulting, Geomorphology,
201, 293–311, 10.1016/j.geomorph.2013.07.003, 2013.Doglioni, C.: A proposal for the kinematic modelling of the W-dipping
subduction – possible applications to the Tyrrhenian-Apennines system,
Terra Nova, 3, 423–434, 10.1111/j.1365-3121.1991.tb00172.x, 1991.
Elter, P., Giglia, G., Tongiorgi, M., and Trevisan, L.: Tensional and
compressional areas in the recent (Tortonian to Present) evolution of the
Northern Apennines, B. Geofis. Teor. Appl., 42, 3–18, 1975.
Faccenna, C., Florindo, F., Funiciello, R., and Lombardi, S.: Tectonic
setting and Sinkhole Features: case histories from Western Central Italy,
Quat. Proc., 3, 47–56, 1993.Faccenna, C., Becker, T. W., Auer, L., Billi, A., Boschi, L., Brun, J.,
Capitanio, F. A., Funiciello, F., Horvàth, F., Jolivet, L., Piromallo,
C., Royden, L., Rossetti, F., and Serpelloni, E.: Mantle dynamics in the
Mediterranean, Rev. Geophys., 52, 28–332, 10.1002/2013RG000444, 2014.
Florea, L. J.: Using State-wide GIS data to identify the coincidence between
sinkholes and geologic structure, J. Cave Karst Stud., 67, 120–124, 2005.
Ford, D. C. and Williams, P.: Karst Hydrogeology and Geomorphology, Wiley,
Chichester, UK, 562 pp., 2007.Frumkin, A. and Raz, E.: Collapse and subsidence associated with salt
karstification along the Dead Sea, Carbonate. Evaporite., 16, 117–130,
10.1007/bf03175830, 2001.Gherardi, F. and Pierotti, L.: The suitability of the Pieve Fosciana
hydrothermal system (Italy) as a detection site for geochemical seismic
precursors, Appl. Geochem., 92, 166–179,
10.1016/j.apgeochem.2018.03.009, 2018.
Giovannetti, F.: Pieve Fosciana Ieri e Oggi, Amministrazione comunale di
Pieve Fosciana, Lucca, Italy, 51 pp., 1975.Gutierréz, F., Guerrero, J., and Lucha, P.: A genetic classification of
sinkholes illustrated from evaporite paleokarst exposures in Spain, Environ.
Geol., 53, 993–1006, 10.1007/s00254-007-0727-5, 2008.Gutierréz, F., Parise, M., De Waele J., and Jourde, H.: A review on
natural and human-induced geohazards and impacts in karst, Earth-Sci. Rev.,
138, 61–88, 10.1016/j.earscirev.2014.08.002, 2014.Hanssen, R. F.: Radar Interferometry: Data Interpretation and Error Analysis,
Kluwer Academic Publisher, 10.1007/0-306-47633-9, 2001.Harris, R. A.: Large earthquakes and creeping faults, Rev. Geophys., 55,
169–198, 10.1002/2016RG000539, 2017.
Harrison, R. W., Newell, W. L., and Necdet, M.: Karstification Along an
Active Fault Zone in Cyprus, U.S. Geological Survey Water-Resources
Investigations Report 02-4174, Atlanta, Georgia, USA, 2002.Istituto Nazionale di Geofisica e Vulcanologia (INGV): Italian Seismological
Instrumental and Parametric Data-Base, ISIDe working group, version 1.0,
10.13127/ISIDe, 2016.Johnson, A. G., Kovach, R. L., and Nur, A.: Pore pressure changes during
creep events on the San Andreas Fault, J. Geophys. Res., 78, 851–857,
10.1029/JB078i005p00851, 1973.Kanamori, H.: The Energy Release in Great Earthquakes, J. Geophys. Res.,
82, 2981–2987, 10.1029/JB082i020p02981, 1977.Kawashima, K., Aydan, O., Aoki, T., Kishimoto, I., Konagal, K., Matsui, T.,
Sakuta, J., Takahashi, N., Teodori, S.-P., and Yashima, A.: Reconnaissance
investigation on the damage of the 2009 L'Aquila, Central Italy earthquake,
J. Earthq. Eng., 14, 817–841, 10.1080/13632460903584055, 2010.Le Breton, E., Handy, M., Molli, G., and Ustaszewski K.: Post-20 Ma Motion
of the Adriatic Plate: New Constraints from Surrounding Orogens and
Implications for Crust-Mantle Decoupling, Tectonics, 36, 3135–3154,
10.1002/2016TC004443, 2017.Linde, A. T., Gladwin, M. T., Johnston, M. J. S., Gwyther, R. L., and Bilham,
R. G.: A slow earthquake sequence on the San Andreas fault, Nature, 383,
65–68, 10.1038/383065a0, 1996.Manzo, M., Ricciardi, G. P., Casu F., Ventura, G., Zeni, G., Borgström
S., Berardino, P., Del Gaudio, C., and Lanari, R.: Surface deformation
analysis in th Ischia Island (Italy) based on spaceborne radar
interferometry, J. Volcanol. Geoth. Res., 151, 399–416,
10.1016/j.jvolgeores.2005.09.010, 2006.Meletti, C., Patacca, E., and Scandone P.: Construction of a Seismotectonic
Model: The Case of Italy, Pure Appl. Geophys., 157, 11–35,
10.1007/PL00001089, 2000.Micklethwaite, S., Sheldon, H. A., and Baker, T.: Active fault and shear
processes and their implications for mineral deposit formation and discovery,
J. Struc. Geol., 32, 151–165, 10.1016/j.jsg.2009.10.009, 2010.Molli, G., Torelli, L., and Storti, F.: The 2013 Lunigiana (Central Italy)
earthquake: Seismic source analysis from DInSar and seismological data, and
geodynamic implications for the northern Apennines. A discussion,
Tectonophysics, 668–669, 108–112, 10.1016/j.tecto.2015.07.041, 2016.
Molli, G., Pinelli, G., Bigot, A., Bennett, R., Malavieille, J., and
Serpelloni, E.: Active Faults in the inner northern Apennines: a
multidisciplinary reappraisal. From 1997 to 2016: Three Destructive
Earthquakes along the Central Apennnine Fault system, Camerino, Italy,
19–22 July 2017, Abstract Volume, p. 29, 2017.Nadeau, R. M., Foxal, W., and McEvilly, T. V.: Clustering and Periodic
Recurrence of Microearthquakes on the San Andreas Fault at Parkfield,
California, Science, 267, 503–507, 10.1126/science.267.5197.503,
1995.Nayak, A. and Dreger, D. S.: Moment Tensor Inversion of Seismic Events
Associated with the Sinkhole at Napoleonville Salt Dome, Louisiana, B.
Seismol. Soc. Am., 104, 1763–1776, 10.1785/0120130260, 2014.Nishiguchi, T., Tsuchiya, S., and Imaizumi, F.: Detection and accuracy of
landslide movement by InSAR analysis using PALSAR-2 data, Landslides, 14,
1483–1490, 10.1007/s10346-017-0821-z, 2017.Nisio, S.: The sinkholes in the Tuscany Region, in: Natural sinkhole
phenomena in the Italian plain area, Memorie descrittive della carta
geologica d'Italia, 85, Istituto Superiore per la Protezione e la Ricerca
Ambientale, Servizio Geologica d'Italia, 213–268, available at:
http://www.isprambiente.gov.it/it/pubblicazioni/periodici-tecnici/memorie-descrittive-della-carta-geologica-ditalia/i-fenomeni-naturali-di-sinkhole-nelle-aree-di
(last access: 11 September 2018), 2008.Nof, R. N., Baer, G., Ziv, A., Raz, E., Atzori, S., and Salvi, S.: Sinkhole
precursors along the Dead Sea, Israel, revealed by SAR interferometry,
Geology, 41, 1019–1022, 10.1130/G34505.1, 2013.Parise, M. and Vennari, C.: A chronological catalogue of sinkholes in Italy:
the first step toward a real evaluation of the sinkhole hazard, in: Sinkholes
and the Engineering and Environmental Impacts of Karst: Proceedings of the
Thirteenth Multidisciplinary Conference, edited by: Land, L., Doctor, D. H.,
and Stephenson, J. B., NCKRI Symposium 2, 6–10 May 2013, Carlsbad, New
Mexico, USA, National Cave and Karst Research Institute, Carlsbad (NM),
10.5038/9780979542275.1149, 2013.Parise, M., Perrone, A., Violante, C., Stewart, J. P., Simonelli, A., and
Guzzetti, F.: Activity of the Italian National Research Council in the
aftermath of the 6 April 2009 Abruzzo earthquake: the Sinizzo Lake case
study, Proc. 2nd Int. Workshop “Sinkholes in the Natural and Anthropogenic
Environment”, 3–4 December 2009, Rome, Italy, 623–641,
10.13140/2.1.3094.1127, 2010.
Patacca, E. and Scandone, P.: Post-Tortonian mountain building in the
Apennines, the role of the passive sinking of a relic lithospheric slab, in:
The Lithosphere in Italy, Advances in Earth Science Research, 5–6 May 1987,
Rome, Itlay, 157–176, 1989.Pepe, A. and Lanari, R.: On the extension of the minimum cost flow algorithm
for phase unwrapping of multitemporal differential SAR interferograms, IEEE
T. Geosci. Remote, 44, 2374–2383, 10.1109/TGRS.2006.873207, 2006.Pezzo, G., Boncori, J. P. M., Atzori, S., Piccinini, D., Antonioli, A., and
Salvi, S.: The 2013 Lunigiana (Central Italy) earthquake: Seismic source
analysis from DInSAR and seismological data, and geodynamical implications
for the northern Apennines, Tectonophysics, 636, 315–324,
10.1016/j.tecto.2014.09.005, 2014.
Pinelli, G.: Tettonica recente e attiva nell'Appennino interno a Nord
dell'Arno: una revisione delle strutture e delle problematiche, Diploma
Thesis, Dipartimento di Scienze della Terra, Università di Pisa, Italy,
89 pp., 2013.
Raffaelli, R.: Sulle acque termali di Pieve Fosciana in Garfagnana, Memoria
diretta al Consiglio Provinciale di Massa, Tipografia dei fratelli Nistri,
Pisa, Italy, 23 pp., 1869.Rau, R., Chen, K. H., and Ching, K.: Repeating earthquakes and seismic
potential along the northern Longitudinal Valley fault of Taiwan, Geophys.
Res. Lett., 34, L24301, 10.1029/2007GL031622, 2007.Regione Toscana: GEOscopio, Direzione urbanistica e poitivhe abitative,
Settore Sistema Informativo Territoriale ed Ambientale, available at:
http://www502.regione.toscana.it/geoscopio/cartoteca.html, last access:
11 September 2018.Rovida, A., Locati, M., Camassi, R., Lolli, B., and Gasperini, P. (Eds.):
CPTI15, the 2015 version of the Parametric Catalogue of Italian Earthquakes,
Istituto Nazionale di Geofisica e Vulcanologia, 10.6092/INGV.IT-CPTI15,
2016.Salamon, A.: Seismically induced ground effects of the February 11, 2004,
ML=5.2, North-eastern Dead Sea earthquake, The Ministry of
National Infrastructures, Geological Survey of Israel, Open File Report
GSI/30/04, 25 pp., available at:
http://www.isprambiente.gov.it/files/progetti/inqua/report-final.pdf
(last access: 11 September 2018), 2004.Santo, A., Del Prete, S., Di Crescenzo, G., and Rotella, M.: Karst processes
and slope instability: some investigations in the carbonate Apennine of
Campania (southern Italy), in: Natural and Anthropogenic Hazards in Karst
Areas: Recognition, Analysis, and Mitigation, edited by: Parise, M. and Gunn,
J., Geological Society, London, 279, 59–72, 10.1144/SP279.6, 2007.Scholz, C. H.: Earthquakes and friction laws, Nature, 391, 37–42,
10.1038/34097, 1998.Serpelloni, E., Anzidei, M., Baldi, P., Casula, G., and Galvani, A.: Crustal
velocity and strain -rate fields in Italy and surrounding regions: New
results from the analysis of permanent and non-permanent GPS networks,
Geophys. J. Int., 161, 861–880, 10.1111/j.1365-246X.2005.02618.x,
2005.Shalev, E. and Lyakhovsky, V.: Viscoelastic damage modelling of sinkhole
formation, J. Struct. Geol., 42, 163–170, 10.1016/j.jsg.2012.05.010,
2012.
Sibson, R. H.: Structural permeability of fluid-driven fault-fracture meshes.
J. Struct. Geol., 18, 1031–1042, 10.1016/0191-8141(96)00032-6, 1996.Stramondo, S., Vannoli, P., Cannelli, V., Polcari, M., Melini, D., Samsonov,
S., Moro, M., Bignami, C., and Saroli, M.: X- and C-band SAR surface
displacement for the 2013 Lunigiana earthquake (Northern Italy): a breached
relay ramp?, IEEE J. Sel. Top. Appl., 7, 2746–2753,
10.1109/JSTARS.2014.2313640, 2014.Tertulliani, A. and Maramai, A.: Macroseismic evidence and site effects for
the Lunigiana (Italy) 1995 Earthquake, J.Seismol., 2, 209–222,
10.1023/A:1009734620985, 1998.U.S. Geological Survey (USGS): EarthExplorer, U.S. Department of the
Interior, available at: https://earthexplorer.usgs.gov, last access:
11 September 2018.Wadas, S. H., Tanner, D. C., Polom, U., and Krawczyk, C. M.: Structural
analysis of S-wave seismics around an urban sinkhole: evidence of enhanced
dissolution in a strike-slip fault zone, Nat. Hazards Earth Syst. Sci., 17,
2335–2350, 10.5194/nhess-17-2335-2017, 2017.Wei, M., Sandwell, D., and Fialko, Y.: A silent Mw 4.7 slip event
of October 2006 on the Superstition Hills fault, southern California, J.
Geophys. Res., 114, B07402, 10.1029/2008JB006135, 2009.Yarushina, V. M., Podladchikov, Y. Y., Minakov, A., and Räss, L.: On the
Mechanisms of Stress-Triggered Seismic Events during Fluid Injection, in:
Sixth Biot Conference on Poromechanics, 9–13 July 2017, Paris, France,
795–800, 10.1061/9780784480779.098, 2017.